A conventional approach to acquiring high quality iris or face images of moving subjects is to freeze the subject motion by using extremely short exposures. The subject must be brightly illuminated by a flash in order to get a well exposed image. This approach commonly breaks down for larger distances because the flash power that is required in order to obtain an acceptable image often becomes eye unsafe.
Producing a well exposed image of a moving subject without flash illumination typically requires extending the image exposure. One drawback with this approach is that extending the image exposure introduces motion blur unless the relative motion between the subject and the camera sensor is eliminated.
The motion of a subject relative to the camera sensor can be real, apparent or both. Real motion is the result of the physical motion of the subject and/or the sensor. Real motion is described by a velocity vector, v(t), which gets decomposed into two components. The axial velocity vector points toward the sensor and is aligned with the optical axis of the sensor. The lateral velocity vector is the velocity vector projection into an X-Y plane perpendicular to the optical axis. Axial velocity affects the focusing of camera optics and may introduce magnification blur for long exposures. Lateral velocity causes motion blur.
Existing systems attempt to estimate the velocity vectors and then move items within the camera in order to compensate for axial and lateral motion of the subject relative to the camera. Moveable lenses are widely used as image stabilizing elements in cameras to counter camera shake during exposure. One known approach is to use a pair of inertial sensors to detect camera motion between and then move the stabilization lens to compensate. An alternative approach employs a tip-tilt minor to compensate for the relative motion.
While the above described solutions can counter motion originating at the camera, they can neither stabilize the projection of a moving object, even if the camera is rock steady, nor can they handle the effects of the atmospheric optical instability. Some components of the shimmer manifest themselves as apparent motion, even if both the object and camera are perfectly stationary.
Another drawback with these types of solutions is that they typically cannot track very fast motions. In addition, these approaches usually utilize delicate mechanical parts that make their use problematic in military and security applications.